Capacitors are used in circuit configurations in pulse power applications, such as for the operation of gas discharge lasers that require faster and higher voltage and higher current pulses. For these applications, in order to obtain some form of voltage multiplication and low inductance high current discharge operation, the LC inversion and the capacitor transfer circuit configurations are commonly chosen as known by those skilled in the art. A brief description of these circuit configurations is given herewith, as well as shown in FIGS. 1 and 2, respectively.
In the LC inversion discharge circuit (FIG. 1), two equal capacitors 1, 2 are charged up in parallel. Upon triggering the switching gap to discharge one of the capacitors, there will be a voltage oscillation leading to a voltage inversion. With a small voltage damping across a switching device such as a spark gap or a rail gap switch 4, the voltage across the two capacitors can experience a near voltage doubling, which is applied across a laser channel 3, resulting in the electrical breakdown of the gas medium. Here it is assumed that the charging element 5 is a resistor of sufficient high resistance or an inductor of appropriate inductance.
In the capacitor transfer circuit (FIG. 2), two unequal capacitors are used. The larger storage capacitor 6 is initially charged to its full voltage and then triggered to discharge across a spark gap switch 9. Part of the stored charges is transferred to the normally 3 to 4 times smaller peaking capacitor 7 leading to a near doubling of the voltage for the discharge across the gas laser channel 8. In order to obtain low circuit inductance operation, the smaller peaking capacitor is placed as close to the discharge channel as possible leading to a fast discharge across the channel. Like in the case of the LC inversion discharge circuit, the charging element 5 is again either a resistor or an inductor.
The three types of capacitors commonly used in circuit configurations in pulse power applications are flat parallel plate capacitors, oil impregnated folded Mylar®/paper foil capacitors and ceramic doorknob capacitors.
The flat parallel plate capacitors operate in atmospheric conditions without immersion in oil normally and use relatively thick Mylar® foils as insulating layers. Used mainly in lasers requiring low operating voltages of 10 to 20 kV and relatively small capacitances of 10 to 25 nF, the two capacitors 15 are constituted of three aluminum plates sandwiching a few layers of Mylar® sheets of appropriate thickness and stacked up on top of one another (see FIG. 3).
The laser channel 14 is connected directly on the edges of the top and bottom electrode plates 15a and 15c of the capacitor, forming low circuit inductance while the spark gap 16 is connected at the opposite end of the plates 15b and 15c. Depending on the magnitude of the two capacitors and their connections to the laser channel and the spark gap, two circuit configurations of the transmission line type LC inversion circuit (commonly known as a Blumlein circuit) or the capacitor transfer circuit configuration can be readily obtained.
Due to the compact circuit discharge loops of such flat plate capacitor discharge, relatively low circuit inductance is achieved. However, the major setbacks are the relatively low operating voltage and small capacitance of the flat plate capacitors. This has limited the maximum discharge current to tens of kiloamperes for a 0.5 m long laser discharge channel, irrespective of whether it is connected using the LC inversion or the capacitor transfer circuit.
On the other hand, the oil impregnated folded Mylar®/paper foil capacitors are available in a large range of capacitances ranging from 15 to 200 nF and rated between 20 to 100 kV.
U.S. Pat. No. 3,711,746 describes the basic methods of folding these capacitors, in particular in FIG. 2 of the US patent. Two aluminum foil electrodes and two sets of dielectric materials are laid alternately one on top of another in the sequence of aluminum, dielectric materials, aluminum and dielectric materials. These are then folded flat to be rectangular or square in dimension for a number of times to constitute a capacitor section. The dielectric materials typically consist of one or two sheets of Mylar® sandwiched by two or three layers of kraft paper. Pairs of aluminum foil tabs are inserted along the sides of the folded capacitors to make electrical contacts with the electrodes. Multiple units of these capacitor sections are then stacked and connected in series by appropriate crimping of the adjoining tabs of the stacked sections to enable higher voltage operation. The finished unit is then thoroughly baked and dried in vacuum before being impregnated in castor oil and appropriately encased.
These capacitors are available commercially in different dimensions in accordance with the capacitance and voltage range. The nominal self inductance of these stand alone capacitor units is typically quoted by the manufacturers to be in the range from 15 to 25 nH for an approximately 12 cm square, 3 cm thick folded foil capacitor unit. Due to this relatively large capacitor self inductance and the corresponding large discharge loop inductance, such commercially available folded foil capacitors are not suitable for use in high peak discharge current LC inversion circuits or as peaking capacitors in capacitor transfer circuits. These stand alone units of folded foil capacitors are normally used as storage capacitors in capacitor transfer circuits.
Ceramic doorknob capacitors (FIG. 4) are fabricated in different sizes ranging in values from 0.1 to 10 nF and rated at 15 to 40 kV. The larger units are often used as storage capacitors with a number of these units connected in parallel. The smaller units are used as peaking capacitors and are normally connected in two arrays along the two sides of the discharge electrodes. Examples of ceramic doorknob capacitors are disclosed on the U.S. Pat. Nos. 4,939,620 and 3,946,290.
Canadian Patent No. 1,287,890 describes the assembly of the 2-stage LC inversion circuit by using a +V and a −V charging across the spark gap resulting in a near fourfold increase in the output voltage with respect to the input voltage.
Four doorknob types ceramic capacitors are used in these 2-stage LC inversion circuits. By arranging two rows of doorknob capacitors symmetrically on the two sides of the electrodes, the current can be doubled, thus the name double sided 2-stage LC inversion circuit. Nevertheless, due to the physical size of the ceramic capacitors, relatively low discharge current of the order of a few tens of kiloamperes for a 35 cm long discharge channel is obtained.
Many attempts have thus been made in the past to scale the discharge voltage as well as to increase the discharge current with various circuit configurations. However, with the presently available choice of capacitors and choice of circuit configurations, the designs of transverse discharge lasers have led to circuits with relatively high inductance and/or relatively low operating voltage. These designs only manage to generate relatively low peak current densities of between 0.5 up to a maximum of 2 to 3 kiloamperes per unit centimeter length or so in a few tens of nanoseconds current pulse.